Abdominal Angiostrongylosis is produced by Angiostrongylus costaricensis, a metastrongylid described in Costa Rica from Human surgical specimens. Early studies on the life history of the parasite allowed the founding of two definitive hosts (Sigmodon hispidus and Rattus rattus) and one intermediate host (the slug Vaginulus plebeius). The present study tends to elucidate the following points: 1. Prevalence of A. costaricensis in its known hosts in different areas in Costa Rica. 2. Investigation of new Hosts. 3. Infection rates and geographical distribution of the new hosts.